It is known, according to German Pat. No. 2,942,223, to direct dust-laden crude gases to a filter bed consisting of coarse granular filter material having an average diameter of approximately 10 mm., located between two spaced, gas-permeable walls. In this system, bars of an electrically conducting material are located inside the filter bed. The filter bed continuously travels downward through the device and is taken out of the system for cleaning purposes. The filter bed has a thickness of 600 mm and the electrically conducting bars are connected to an electrical potential of a maximum of 50,000 volts.
The dust separation inside the filter bed is to some extent improved by the array of electrically conducting bars. However, during operation of this known system, it becomes apparent that certain disadvantages must be accepted. These disadvantages can be summarized as follows.
(1) During operation, the granular filter material continuously moves downwardly and is discharged to the outside of the system and returned to the filter bed from above, after cleaning. An elaborate cleaning unit, for example, cloth tube filters and the associated conveying devices are required. These requirements make the complete system considerably more expensive.
(2) Since the electrically conducting elements are arrayed in the mass of filter bed and the transfer of current must not take place from one of the gas-permeable walls to the electrically conducting elements, the distance of these elements from the two gas-permeable walls must be greater than the sparkover distance corresponding to the high potential being applied to the filter bed. Under most circumstances, the thickness of the filter bed must correspond to at least twice the sparkover distance. Such increase in the thickness of the filter bed requires enormous quantities of granular filter material. In a typical industrial filter having a height of approximately 10 meters, this would amount to 150 tons of granulated filter material which must be circulated and cleaned once every 4 hours.
(3) The coarse-grained granular filter material of the filter bed of the prior art allows the dust contained in the dust laden crude gas to penetrate to a greater depth into the filter bed. This requires a filter bed thickness of approximately 600 mm. for a satisfactory dust removal.
(4) Dust laden crude gas flows transversely through the granular filter material which is moving downwardly and is continuously being replenished, from above, by a new clean granular filter material. Accordingly, the lowest portion of the filter bed is always the most heavily contaminated thereby causing this portion to be electrically conductive. This conductivity causes a continuous flow of an electric current in this lower region. Obviously, operating expenditure is considerably increased. Furthermore, in this type of arrangement, a higher quantity of gas moves through the upper region of the filter bed while undergoing a lower degree of cleaning. The efficiency of the system for the cleaning of the dust laden crude gas increases in the downward direction in accordance with the increased contamination of the filter bed. Although excellent dust separation could be possible in the lower section of the filter bed, the amount of gas effectively passing through the section is very low, virtually equal to zero, due to a heavy contamination.
In this known system, the choice is either to depart considerably from the optimum operating conditions or, for the sake of safety, to increase the layer thickness to an extreme degree. However, this presents further disadvantages.
(5) In consideration of the large mass of the granular filter material and its correspondingly high heat-absorption capacity, the filter material must normally be preheated before entering the filter bed. This is to reduce undesired condensation to a permissible amount. Condensation causes the baking together of the dust particles and the formation of dust cakes, known as moon pies, which are impermeable to gas. The expenditure of energy associated with this warming-up process is considerable. Condensation is virtually unavoidable during this process and leads to sparkovers of the high potential.